As a digital modulation scheme of a wireless apparatus, quadrature modulation may be broadly classified into an analog scheme and a digital scheme. The analog scheme is advantageous in that relatively inexpensive implementation can be achieved; a distortion compensation technique of baseband feedback such as Cartesian is easily applicable; and direct conversion to a desired frequency can be achieved. On the other hand, it is disadvantageous in that it is necessary to adjust amplitude balance, orthogonality, and DC. The digital scheme is advantageous in that it is not necessary to adjust amplitude balance, orthogonality, and DC by using digital quadrature modulation. However, the cost is increased since a high-speed digital-to-analog converter (DAC) is required, and thus it is difficult to adopt direct conversion in the digital scheme.
Generally, the analog quadrature modulation is preferred in narrow-band wireless communications, since Cartesian technology by which an amplification can be achieved with high efficiency, low distortion and low cost may be used. Therefore, analog quadrature modulation may be employed in many cases. Further, recently, in view of realizing a high efficiency operation in the power amplifier for low power consumption and miniaturization of the wireless apparatus, there has been used a wireless technology using a power amplification device of an envelope tracking (ET) scheme.
As for the wireless transmission technology using the power amplification device of the ET scheme, various techniques have been proposed. For example, there is a technique to provide an ET power amplification device which not only reduces the non-linear distortion of a power amplifier but also has high power efficiency (see, e.g., Patent Document 1). Specifically, it is configured to detect an amplitude of an input signal, output a control signal based on amplitude-power supply voltage characteristics, and supply the power supply voltage according to the control signal to the power amplifier. Further, in the amplitude-power supply voltage characteristics, the power supply voltage varies linearly with respect to the amplitude and the lower limit is set to the power supply voltage.
Patent Document 1: Japanese Patent Application Publication No. 2013-55449
Here, it is significant to remove the DC signal which is considered as one of the undesired waves included in the output signal of the wireless apparatus. In the wireless apparatus equipped with an analog quadrature modulation function, the DC removal is required to be completed before the transmission of the actual modulation signal. FIG. 1 shows a sequence when applied to a transceiver. When a PTT (Push To Talk) button is pressed, a DC removal process is performed and then the modulation signal is transmitted.
In the ET operation in general, an envelope is calculated from an IQ signal by a digital Signal processor (DSP) or the like, and a desire voltage is applied to a power amplifier while varying the applied voltage. The supply of the voltage to the power amplifier is performed by a switching amplifier such as class D amplifier.
In this regard, when, for example, a Cartesian loop is employed as a distortion compensation function, the DC removal needs to be performed with the configuration of the feedback loop since the Cartesian loop has a feedback-type configuration. In other words, it may be required that the wireless transmission unit is active, that is, the power amplifier is in an ON state. However, since the power amplifier is in an OFF state and further no IQ signal is present during DC calibration or the like, an envelope output is zero. Therefore, the feedback loop is in an OFF state at the timing of the DC removal, and a linearizer's operation is not performed. That is, there is a problem that appropriate DC removal cannot be performed.
The technique disclosed in Patent Document 1 can effectively solve non-linear distortion of the power amplifier. However, it is required to develop another technique since enough consideration is not made with respect to the above-described problem.
The present disclosure has been made in view of the above circumstances and has an object to solve the above-described problem.